FEMA 453 / May 2006
Risk Management Series
Safe Rooms and Shelters
Protecting People Against Terrorist Attacks
Chapter 2: Structural Design Criteria
2.1 Overview
This chapter discusses explosive threat parameters and measures needed to
protect shelters from blast effects. Structural systems and building envelope
elements for new and existing shelters are analyzed; shelters and FEMA model
building types are discussed; and protective design measures for the defined
building types are provided, as are design guidance and retrofit issues. The
purpose of this chapter is to offer comprehensive information on how to improve
the resistance of shelters when exposed to blast events.
2.2 Explosive Threat Parameters
A detonation involves supersonic combustion of an explosive material and the
formation of a shock wave. The three parameters that primarily determine the
characteristics and intensity of blast loading are the weight of explosives, the
type of the explosives, and the distance from the point of detonation to the
protected building. These three parameters will primarily determine the
characteristics and intensity of the blast loading. The distance of the
protected building from the point of explosive detonation is commonly referred
to as the stand-off distance. The critical locations for detonation are taken to
be at the closest point that a vehicle can approach, assuming that all security
measures are in place. Typically, this would be a vehicle parked along the curb
directly outside the facility, or at the vehicle access control gate where
inspection takes place. Similarly, a critical location may be the closest point
that a hand carried device can be deposited.
There is also no way to effectively know the size of the explosive threat.
Different types of explosive materials are classified as High Energy and Low
Energy and these different classifications greatly influence the damage
potential of the detonation. High Energy explosives, which efficiently convert
the material’s chemical energy into blast pressure, represent less than 1
percent of all explosive detonations reported by the FBI Bomb Data Center. The
vast majority of incidents involve Low Energy devices in which a significant
portion of the explosive material is consumed by def­lagration, which is a
process of subsonic combustion that usually propagates through thermal
conductivity and is typically less destructive than a detonation. In these
cases, a large portion of the material’s chemical energy is dissipated as
thermal energy, which may cause fires or thermal radiation damage.
For a specific type and weight of explosive material, the intensity of blast
loading will depend on the distance and orientation of the blast waves relative
to the protected space. A shock wave is characterized by a nearly instantaneous
rise in pressure that decays exponentially within a matter of milliseconds,
which is followed by a longer term but lower intensity negative phase. The
initial magnitude of pressure is termed the peak pressure and the area under a
graph of pressure plotted as a function of time, also known as the airblast
pressure time history, is termed the impulse (see Figure 2-1). Therefore, the
impulse associated with the shock wave considers both the pressure intensity and
the pulse duration. As the front of the shock-wave propagates away from the
source of the detonation at supersonic speed, it expands into increasingly
larger volumes of air; the peak incident pressure at the shock front decreases
and the duration of the pressure pulse increases. The magnitude of the peak
pressures and impulses are reduced with distance from the source and the
resulting patterns of blast loads appear to be concentric rings of diminishing
intensity. This effect is analogous to the circular ripples that are created
when an object is dropped in a pool of water. The shock front first impinges on
the leading surfaces of a building located within its path and is reflected and
diffracted, creating focus and shadow zones on the building envelope. These
patterns of blast load intensity are complicated as the waves engulf the entire
building. The pressures that load the roof, sides, and rear of the building are
termed incident pressures, while the pressures that load the building envelope
directly opposite the explosion are termed reflected pressures. Both the
intensity of peak pressure and the impulse affect the hazard potential of the
blast loading. A detailed analysis is required to determine the magnitude of
pressure and impulse that may load each surface relative to the origin of the
detonation.
Figure 2-1: Airblast pressure time history chart showing the initial magnitude
of pressure, which is termed the peak pressure, and the area under a graph of
pressure plotted as a function of time, also known as the airblast
pressure time history, which is termed the impulse.
The thresholds of different types of injuries associated with damage to wall
fragments and/or glazing are depicted in Figure 2-2. This range to effects chart
shows a generic interaction between the weight of the explosive threat and its
distance to an occupied building. These generic charts, for conventional
construction, provide information to law enforcement and public safety officials
that allow them to establish safe evacuation distances should an explosive
device be suspected or detected. However, these distances are so site-specific
that the generic charts provide little more than general guidance in the absence
of more reliable site-specific information. Based on the information provided in
the chart, the onset of significant glass debris hazards is associated with
stand-off distances on the order of hundreds of feet from a vehicle-borne
explosive detonation while the onset of column failure is associated with stand-
off distances on the order of tens of feet.
Figure 2-2: Range to effects chart showing the minimum stand-off (ft) and the
weapon yield (lbs-TNT). The graph compares the explosive environment of luggage,
automobiles, vans, and trucks.
Source: Defense Threat Reduction Agency
2.2.1 Blast Effects in Low-rise Buildings
Many shelters can be part of low-rise buildings. Although small weights of
explosives are not likely to produce significant blast loads on the roof, low-
rise buildings may be vulnerable to blast loadings resulting from large weights
of explosives at large stand-off distances that may sweep over the top of the
building. The blast pressures that may be applied to these roofs are likely to
far exceed the conventional design loads and, unless the roof is a concrete deck
or concrete slab structure, it may fail. There is little that can be done to
increase the roof’s resistance to blast loading that doesn’t require extensive
renovation of the building structure. Figure 2-3 shows the ever expanding blast
wave as it radiates from the point of detonation and causes, in sequence of
events, the building envelope to fail, the internal uplift on the floor slabs,
and eventually the engulfment of the entire building.
[Begin Figure]
Figure 2-3: Blast damage to buildings.
1. Blast wave breaks windows, exterior walls blown in, and columns may be
damaged.
2. Blast wave forces floors upward.
3. Blast wave surrounds structure, downward pressure on roof, and inward
pressure on all sides.
SOURCE: Naval Facilities Engineering Service Center, User’s Guide on Protection
Against Terrorist Vehicle Bombs, May 1998
[End Figure}
In addition to the blast pressures that may be directly applied to the exterior
columns and spandrel beams, the forces collected by the building envelope will
be transferred through the slabs to the structural frame or shear walls that
transfer lateral loads to the foundations. The extent of damage will be greatest
in close proximity to the detonation; however, depending on the intensity of the
blast, large inelastic deformations will extend throughout the building and
cause widespread cracking to structural and nonstructural partitions.
In addition to the hazard of impact by building envelope debris propelled into
the building or roof damage that may rain down, the occupants may also be
vulnerable to much heavier debris resulting from structural damage. Progressive
collapse occurs when an initiating localized failure causes adjoining members to
be over­loaded and fail, resulting in a cascading sequence of damage that is
disproportionate to the originating extent of localized failure. The initiating
localized failure may result from a sufficiently sized parcel bomb that is in
contact with a critical structural element or from a vehicle sized bomb that is
located a short distance from the building (see Figure 2-4). However, a large
explosive device at a large stand-off distance is not likely to selectively fail
a single structural member; any damage that results from this scenario is more
likely to be widespread and the ensuing collapse cannot be considered
progressive. Although progressive collapse is not typically an issue for
buildings three stories or shorter, transfer girders and non-ductile, non-
redundant construction may produce structural systems that are not tolerant of
localized damage conditions. The columns that support transfer girders and the
transfer girders themselves may be critical to the sta­bility of a large area of
floor space.
Figure 2-4: Alfred P. Murrah Federal Office Building. A photo of the building
after the blast.
SOURCE: U.S. Air Force
As an example, panelized construction that is sufficiently tied together can
resist the localized damage or large structural deformations that may result
from an explosive detonation. Although the explosive detonation opposite the
Khobar Towers destroyed the exterior façade, the panelized structure was
sufficiently tied together to permit relatively large deformations without loss
of structural stability (see Figure 2-5). This highlights the benefits of
ductile and redundant detailing for all types of construction.
Figure 2-5: Khobar Towers. A photo of the building after the blast.
SOURCE: U.S. Air Force
To mitigate the effects of in-structure shock that may result from the infilling
of blast pressures through damaged enclosures, nonstructural overhead items
should be located below the raised floors or tied to the ceiling slabs with
seismic restraints. Nonstructural building components, such as piping, ducts,
lighting units, and conduits must be sufficiently tied back to the building to
prevent failure of the services and the hazard of falling debris.
The contents of this manual supplement the information provided in FEMA 361,
Design and Construction Guidance for Community Shelters and FEMA 320, Taking
Shelter From the Storm: Building a Safe Room Inside Your House. Although this
publication does not specifically address nuclear explosions and shelters that
protect against radiological fallout, this information may be found in FEMA TR-
87, Standards for Fallout Shelters. The contents of FEMA 452, A How-To Guide to
Mitigate Potential Terrorist Attacks Against Buildings will help the reader
identify critical assets and functions within buildings, determine the threats
to these assets, and assess the vul­nerabilities associated with those threats.
2.2.2 Blast Effects in High-rise Buildings: The Urban Situation
High-rise buildings must resist significant gravity and lateral load effects;
although the choice of framing system and specific structural details will
determine the overall performance, the lower floors, which are in closest
proximity to a vehicle-borne threat, are inherently robust and more likely to be
resistant to blast loading than smaller buildings. However, tall buildings are
likely to be located in dense urban environments that tend to trap the blast
energy within the canyon like streets as the blast waves reflect off of
neighboring structures. Furthermore, tall buildings are likely to contain
underground parking and loading docks that can introduce significant internal
explosive threats. While these internal threats may be prevented through
rigorous access control procedures, there are few conditions in which vehicular
traffic can be restricted on city streets. Anti-ram streetscape elements are
required to maintain a guaranteed stand-off distance from the face of the
building.
In addition to the hazard of structural collapse, the façade is a much more
fragile component. While the lower floor façade is likely to fail in response to
a sizable vehicle threat at a sidewalk’s distance from the building, the peak
pressures and impulses at higher elevations diminish due to the increased stand-
off distance and the associated shallow angle of incidence (measured with
respect to the vertical height of the building). Although reflections off of
neighboring structures are likely to affect the intensity of blast loads, the
façade loads at the upper floors will be considerably lower than the loads at
the lower floors and the extent of façade debris will reflect this. A detailed
building-specific analysis of the structure and the façade is required to
identify the inherent strengths and vulnerabilities. This study will indicate
the safest place to locate the shelter.
2.3 Hardened Construction
2.3.1 Structural System
A shelter will only be effective if the building in which it is located remains
standing. It is unreasonable to design a shelter within a building with the
expectation that the surrounding structure may collapse. Although the shelter
must be able to resist debris impact, it is not reasonable for it to withstand
the weight of the building crashing down upon it. Therefore, the effectiveness
of the shelter will depend on the ability of the building to sustain damage, but
remain standing. The ability of a building to withstand an explosive event and
remain standing depends on the characteristics of the structure. Some of these
characteristics include:
- Mass. Lightweight construction may be unsuitable for providing resistance to
blast loading. Inertial resistance may be required in addition to the strength
and ductility of the system.
- Shear capacity. Shear is a brittle mode of failure and primary members and/or
their connections should therefore be designed to prevent shear failure prior to
the development of the flexural capacity.
- Capacity for resisting load reversals. In response to sizable blast loads,
structural elements may undergo multiple cycles of large deformation. Similarly,
some structural elements may be subjected to uplift pressures, which oppose
conventional gravity load design. The effects of rebound and uplift therefore
require blast-resistant members to be designed for significant load reversals.
Depending on the cable profile, pre-tensioned or post-tensioned construction may
provide limited capacity for abnormal loading patterns and load reversals.
Draped tendon systems designed for gravity loads may be problematic; however,
the higher quality fabrication and material properties typical for precast
construction may provide enhanced performance of precast elements designed and
detailed to resist uplift and rebound effects resulting from blast loading.
Seated connection systems for steel and precast concrete systems must also be
designed and detailed to accommodate uplift forces and rebound resulting from
blast loads. The use of headed studs is recommended for affixing concrete fill
over steel deck to beams for uplift resistance.
- Redundancy. Multiple alternative load paths in the vertical-load-carrying
system allow gravity loads to redistribute in the event of failure of structural
elements.
- Ties. An integrated system of ties in perpendicular directions along the
principal lines of structural framing can serve to redistribute loads during
catastrophic events.
- Ductility. Structural members and their connections may have to maintain their
strength while undergoing large deformations in response to blast loading. The
ability of a member to develop inelastic deformations allows it to dissipate
considerable amounts of blast energy. The ratio of a member’s maximum inelastic
deformation to a member’s elastic limit is a measure of its ductility. Special
detailing is required to enable buildings to develop large inelastic
deformations (see Figure 2-6).
Figure 2-6: Ductile detailing of reinforced concrete structures. An illustration
showing detail of structural members and their connections.
Historically, cast-in-place reinforced concrete was the preferred material for
explosion-mitigating construction. This is the mate­rial used for military
bunkers, and the military has performed extensive research and testing of its
performance. Among its benefits, reinforced concrete has significant mass,
which improves its inertial resistance; it can be readily proportioned for
ductile behavior and may be detailed to achieve continuity between members.
Finally, concrete columns are less susceptible to global buckling in the event
of the loss of a floor system. However, steel may be similarly detailed to take
advantage of its inherent ductility and connections may be designed to provide
continuity between members. Similarly, panelized precast concrete systems can be
detailed to permit significant deformations in response to explosive loading, as
demonstrated by the performance of Khobar Towers.
Protective design further requires the system to accept localized failure
without precipitating a collapse of a greater extent of the structure. By
allowing the building to bridge over failed components, building robustness is
greatly improved and the unintended consequences of extreme events may be
mitigated. However, it may not be possible for existing construction to be
retrofitted to limit the extent of collapse to one floor on either side of a
failed column. If the members are retrofitted to develop catenary behavior, the
adjoining bays must be upgraded to resist the large lateral forces associated
with this mode of response. This may require more extensive retrofit than is
either feasible or desirable. In such a situation, it may be desirable to
isolate the collapsed region rather than risk propagating the collapse to
adjoining bays. The retrofit of existing buildings to protect against a
potential progressive collapse resulting from extreme loading may therefore best
be achieved through the localized hardening of vulnerable columns. These columns
need only be upgraded to a level of resistance that balances the capacities of
all adjacent structural elements. At greater blast intensities, the resulting
damage would be extensive and create global collapse rather than progressive
collapse. Attempts to upgrade the building to conform to the alternate path
approach would be invasive and potentially counterproductive.
2.3.2 Loads and Connections
Because the shelter will likely suffer significant damage in response to extreme
loading conditions, the shelter must be able to withstand both the direct
loading associated with the natural or manmade hazard and the debris associated
with the damaged building within which it is housed.
Structural systems that provide a continuous load path that supports all
vertical and lateral loads acting on a building are preferred. A continuous load
path ties all structural components together and the fasteners used in the
connections must be capable of developing the full capacity of the members. In
order to provide comprehensive protection, the capacity of each component must
be balanced with the capacity of all other components and the connection details
that tie them together. Because all applied loads must eventually be transferred
to the foundations, the load path must be continuous from the uppermost
structural component to the ground.
After the appropriate loads are calculated for the shelter, they should be
applied to the exterior wall and roof surfaces of the shelter to determine the
design forces for the structural and nonstructural elements. The continuous load
path carries the loads acting on a building’s exterior façade and roof through
the floor diaphragms to the gravity load-bearing system and lateral load-bearing
system. The individual components of the façade and roof must be able to develop
these extraordinary forces, though deformed, and transfer them to the underlying
beams, trusses, girders, shear walls, and columns that provide the global
structural resistance. These structural systems must also be able to develop
uplift forces and load reversals that may accompany these extreme loading
conditions. Uplift forces and load reversals are typically applied contrary to
the conventional design loads and, therefore, details must be developed that
account for these contrary patterns of deformation (see Figure 2-7). Seismic
detailing that addresses ductile behavior despite multiple cycles of load
reversals are generally well suited for all of these extreme loading conditions
and building-specific details must consider each threat condition. Some
construction materials, however, are better suited to developing a load path
that can withstand loads from multiple directions and events. Cast-in-place
reinforced concrete and steel moment frame construction is more commonly
detailed to provide load paths than in "progressive collapse" designs utilizing
panelized or ma­sonry load-bearing construction. Nevertheless, appropriate
details must be developed for nearly all structural systems.
Figure 2-7: Effects of uplift and load reversals. Illustration shows how the
blast pressure weakens the slab column connection.
Floor slabs are typically designed to resist downward gravity loading and have
limited capacity to resist uplift pressures or the upward deformations
experienced during load reversals that may precipitate a flexural or punching
shear failure (see Figure 2-8). Therefore, floor slabs that may be subjected to
significant uplift pressures, such that they overcome the gravity loads and
subject the slabs to reversals in curvature, require additional reinforcement.
If the slab does not contain this tension reinforcement, it must be supplemented
with a lightweight carbon fiber application that may be bonded to the surface at
the critical locations. Carbon fiber reinforcing mats bonded to the top surface
of slabs would strengthen the floors for upward loading and reduce the
likelihood of slab collapse from blast infill uplift pressures as well as
internal explosions in mailrooms or other susceptible regions. This lightweight
high tensile strength material supplements the limited capacity of the concrete
to resist these unnatural loading conditions. An alternative approach would be
to notch grooves in the top of concrete slabs and epoxy carbon fiber rods into
grooves; although this approach may offer a greater capacity, it is much more
invasive.
Figure 2-8: Flat slab failure mechanisms. An illustration showing a flat slab
with columns, and the failure lines.
Similarly, adequate connections must be provided between the roof sheathing and
roof structure to prevent uplift forces from lifting the roof off of its
supports. Reinforcing steel, bolts, steel studs, welds, screws, and nails are
used to connect the roof decking to the supporting structure. The detailing of
these connections depends on the magnitude of the uplift or catenary forces that
may be developed. The attachment of precast planks to the supporting structure
will require special attention to the connection details. However, as with all
other forms of construction, ductile and redundant detailing will produce
superior performance in response to extreme loading.
Wall systems are typically connected to foundations using anchor bolts,
reinforcing steel and imbedded plate systems properly welded together, and
nailed mechanical fasteners for wood construction. Although these connections
benefit from the weight of the structure bearing against the foundations and the
lateral restraint provided by keyed details, the connections must be capable of
developing the design forces in both the connectors and the materials into which
the connectors are anchored.
2.3.3 Building Envelope
Façade components that must transfer the collected loads to the structural
system must be designed and detailed to absorb significant amounts of energy
associated with the extreme loading through controlled deformation. The duration
of the extreme loading significantly influences the criteria governing the
design of the building envelope systems. Significant inelastic deformations may
be permitted for extraordinary events that impart the extreme loading over very
short periods of time (e.g., explosive detonations). The building envelope
system need only be designed to resist the direct shock wave, rebound, and any
reflections off of neighboring buildings, all of which will occur within a
matter of milliseconds (see Figure 2-9).
Figure 2-9: Blast damaged façade
Resistance to blast is often compared to resistance to natural hazards with the
expectation that the protection against one will provide protection against the
other. Therefore, as a first step, one should consider any inherent resistance
derived from a building’s design to resist environmental loading. Extreme wind
loading resulting from tornadoes may similarly be of short enough duration to
permit a large deformation of the façade in response to the peak loading.
Certainly, the debris impact criteria will be similar to that for blast loading.
However, hurricane winds may persist for extended periods of time and the
performance criteria for façade components in response to these sustained
pressures permit smaller deformations and less damage to the system. Breach of
the façade components would permit pressures to fill the building and loads to
be applied to nonstructural components. Anchorages and connections must be
capable of holding the façade materials intact and attached to the building.
Brittle modes of failure must be avoided to allow ductile deformations to occur.
2.3.4 Forced Entry and Ballistic Resistance
Ballistic-resistant design involves both the blocking of sightlines to conceal
the occupants and the use of ballistic-resistant materials to minimize the
effectiveness of the weapon. To reduce exposure, the safe room should be located
as far as possible into the interior of the facility and walls should be
arranged to eliminate sightlines through doorways. In order to provide the
required level of resistance, the walls must be constructed using the
appropriate thickness of ballistic-resistant materials, such as reinforced
concrete, masonry, mild steel plate, or composite materials. The required
thickness of these materials depends on the level of ballistic resistance;
however, resistance to a high level of ballistic threat may be achieved using
6.5 inches of reinforced concrete, 8 inches of grouted concrete masonry unit
(CMU) or brick, 1 inch mild steel plate, or ¾ inch armor steel plate. A ½-inch
thick layer of bullet-resistant fiberglass may provide resistance up to a medium
level of ballistic threat. Bullet-resistant doors are required for a high level
of protection; however, hollow steel or steel clad doors with pressed steel
frames may be used with an appropriate concealed entryway. Ballistic-resistant
window assemblies contain multiple layers of laminated glass or polycarbonate
materials and steel frames. Because these assemblies tend to be both heavy and
expensive, their number and size should be minimized. Roof structures should
contain materials similar to the ballistic-resistant wall assemblies. Ratings of
bullet-resisting materials are presented in Table 2-1.
[Begin Table]
Table 2-1: UL 752 Ratings of Bullet-resisting Materials
Rating: Level 1
Ammunition: 9 mm full metal copper jacket with lead core
Grain: 124
Minimum Velocity (fps): 1,185
Rating: Level 2
Ammunition: .357 Magnum jacketed lead soft point
Grain: 158
Minimum Velocity (fps): 1,250
Rating: Level 3
Ammunition: .44 Magnum lead semi-wadcutter gas checked
Grain: 240
Minimum Velocity (fps): 1,350
Rating: Level 4
Ammunition: .30 caliber rifle lead core soft point
Grain: 180
Minimum Velocity (fps): 2,540
Rating: Level 5
Ammunition: 7.62 mm rifle lead core full metal copper jacket, military ball
Grain: 150
Minimum Velocity (fps): 2,750
Rating: Level 6
Ammunition: 9 mm full metal copper jacket with lead core
Grain: 124
Minimum Velocity (fps): 1,400
Rating: Level 7
Ammunition: 5.56 mm rifle full metal copper jacket with lead core
Grain: 55
Minimum Velocity (fps): 3,080
Rating: Level 8
Ammunition: 7.62 mm rifle lead core full metal copper jacket, military ball
Grain: 150
Minimum Velocity (fps): 2,750
UL = Underwriters Laboratories
[End Table]
Forced entry resistance is measured in the time it takes for an aggressor to
penetrate the enclosure using a variety of hand tools and weapons. The required
delay time is based on the probability of detecting the aggressors and the
probability of a response force arriving within a specified amount of time. The
different layers of defense create a succeeding number of security layers that
are more difficult to penetrate, provide additional warning and response time,
and allow building occupants to move into defensive positions or designated safe
haven protection (see Figure 2-10). The rated delay time for each component
comprising a defense layer (walls, doors, windows, roofs, floors, ceilings, and
utility openings) must be known in order to determine the effective delay time
for the safe room. Conventional construction offers little resistance to most
forced entry threat severity levels and the rating of different forced entry-
resistant materials is based on standardized testing under laboratory
conditions.
Figure 2-10: Layers of defense. An illustration showing the first, second, and
third layers of defense on a property’s entry control point, and the perimeter
(site property line or fence).
2.4 New Construction
The design of new buildings to contain shelters provides greater opportunities
than the retrofit of existing buildings. Whether the entire building or just the
shelter is to be resistant to the explosive terrorist threat may have a
significant impact on the architectural and structural design of the building.
Furthermore, unless the building is required to satisfy an established security
design criteria, the weight of explosive that the building is to be designed to
resist must be established by a site-specific threat and risk assessment. Even
so, given the evolving nature of the terrorist threat, it is impossible to
predict all the extreme conditions to which the building may be exposed over its
life. Therefore, even if the building is not to be designed to resist any
specific explosive threat, the American Society of Civil Engineers Minimum
Design Loads for Buildings and Other Structures (ASCE-7) requires the building
to be designed to sustain local damage without the building as a whole “being
damaged to an extent disproportionate to the original local damage.” The
building can therefore be designed to prevent the progression of collapse in the
unlikely event a primary member loses its load carrying capacity. This minimum
design feature, achieved through the indirect prescriptive method or direct
alternate path approach, will improve the structural integrity and provide an
additional measure of safety to occupants. Incorporating continuity, redundancy,
and ductility into the design will allow a damaged building to bridge over a
failed element and redistribute loads through flexure or catenary action. This
will limit the extent of debris that might otherwise rain down upon the hardened
shelter. Where specific threats are defined, the vulnerable structural
components may be hardened to withstand the intensity of explosive loading. The
local hardening of vulnerable components in addition to the indirect
prescriptive detailing of the structural system to bridge over damaged
components will provide the most protection to the building.
2.4.1 Structure
Both steel frame and reinforced concrete buildings may be designed and detailed
to resist the effects of an exterior vehicle explosive threat and an interior
satchel explosion. Although steel construction may be more efficient for many
types of loading, both conventional and unconventional, cast-in-place reinforced
concrete construction provide an inherent continuity and mass that makes it
desirable for blast-resistant buildings.
Reinforced concrete is a composite material in which the concrete provides the
primary resistance to compression and shear and the steel reinforcement provides
the resistance to tension and confines the concrete core. In addition to ductile
detailing, which allows the reinforced concrete members to sustain large
deformations and uncharacteristic reversals of curvature, the structural
elements are typically stockier and more massive than their steel frame
counterparts. The additional inertial resistance as well as the continuity of
cast-in-place construction facilitates designs that are capable of sustaining
the high intensity and short duration effects of close-in explosions.
Furthermore, reinforced concrete buildings tend to crack and dissipate large
amounts of energy through internal damping. This limits the extent of rebound
forces and deformations.
Blast-resistant detailing requires continuous top and bottom re­inforcement with
tension lap splices staggered over the spans, confinement of the plastic hinge
regions by means of closely spaced ties, and the prevention of shear failure
prior to developing the flexural capacity (see Figure 2-11). One- or two-way
slabs supported on beams provide the best resistance to near contact satchel
threats, which may produce localized breach, but allow the structure to
redistribute the gravity loads. Concrete columns must be confined with closely
spaced spiral ties, steel jackets, or composite wraps. This confinement
increases the shear resistance, improves the ductility, and protects against the
shattering effects resulting from a near contact explosion. Cast-in-place
exterior walls or precast panels are best able to withstand a sizable stand-off
vehicular explosive threat and may be easily detailed to interact with the
reinforced concrete frame as part of the lateral load-resisting system.
Figure 2-11 Multi-span slab splice locations. An illustration showing continuous
top and bottom reinforcement with tension lap splices staggered over the span.
Steelwork is generally better suited to resist relatively low intensity, but
long duration effects of large stand-off explosions. Steel is an inherently
ductile material that is capable of sustaining large deformations; however, the
very efficient thin-flanged sections make the conventional frame construction
vulnerable to localized damage. Complex stress combinations and concentrations
may occur that cause localized distress and prevent the section from developing
its ultimate strength. Steel buildings may experience significant rebound and
must therefore be designed to support significant reversals of loading. Concrete
filled tube sections or concrete encased flanged sections may be used to protect
the thin-flanged sections and supplement the inertial resistance. Concrete
encasement should extend a minimum of 4 inches beyond the width and depth of the
steel flanges and reinforcing bars may be detailed to tie into the concrete
slabs.
To allow the concrete encasement to be tied into the floor slabs, the typical
metal pan with concrete deck construction will require special detailing. Metal
deck construction provides a spall shield to the underside of the slabs, which
provides additional protection to a near contact satchel situated on a floor.
However, the internal explosive threat will also load the ceiling slabs from
beneath and the beams must contain an ample amount of studs, which far exceeds
the requirements for conventional gravity design, to transfer the slab reactions
to the steel supports without pulling out. If the slabs are adequately connected
to the steel-framing members, these beams will be subjected to abnormal
reversals of curvature. These reversals will subject the mid-span bottom flanges
to transient compressive stress and may induce a localized buckling. Because the
blast loads are transient, the dominant gravity loads will eventually restore
the mid-span bottom flange to tension; however, unless it is adequately braced,
the transient buckling will produce localized damage.
The concrete encasement of the steel beams will provide torsional resistance to
the cross-section and minimize the need for intermediate bracing. If the depth
of the composite section is to be minimized by embedding the steel section into
the thickness of the slab, the slab reinforcement must either be welded to the
webs or run through holes drilled into the webs in order to maintain continuity.
All welding of reinforcing steel must be in accordance with seismic detailing to
prevent brittle failures. Steel columns require full moment splices and the
relatively thin flange sections require concrete encasement to prevent localized
damage. To take full advantage of the steel capacity and dissipate the greatest
amount of energy through ductile inelastic deformation, the beam to column
connections must be capable of developing the plastic flexural capacity of the
members. Connection details, similar to those used in seismic regions, will be
required to develop the corresponding flexural and shear capacity (see Figure 2-
12). Connecting exterior cast-in-place reinforced concrete walls to the steel
frame will require details that transfer both the direct blast loads in bearing
and the subsequent rebound effects in tension. Precast panels are simply
supported at the ends and, unless they span over multiple floors, they lack the
continuity of monolithic cast-in-place wall construction. Cold joints in the
cast-in-place construction require special detailing and the connection details
for the precast panels must be able to resist both the direct blast loads in
bearing and the subsequent rebound effects in tension.
Figure 2-12: Typical frame detail at interior column. An illustration showing
connection details.
Regardless of the materials, framed buildings perform best when column spacing
is limited and the use of transfer girders is limited. Bearing wall systems that
rely on interior cross-walls will benefit from periodically spaced longitudinal
walls that enhance stability and control the lateral progression of damage.
Bearing wall systems that rely on exterior walls will benefit from periodically
spaced perpendicular walls or substantial pilasters that limit the extent of
wall that is likely to be affected.
Free-standing columns do not have much surface area; therefore, air-blast loads
on columns are limited by clear-time effects in which relief waves from the free
edges attenuate the reflected intensity of the blast loads. Where the exterior
façade inhibits clear-time effects prior to façade failure, the columns will
receive the full intensity of the reflected blast pressures. Large stand-off
explosive threats may produce large inelastic flexural deformations that could
initiate P-delta induced instabilities. Short stand-off explosive threats may
cause shear, base plate, or column splice failures. Near contact threats may
cause brisance, which is the shattering of reinforced concrete sections.
Confinement of reinforced concrete members by means of spiral reinforcement,
steel jackets, or carbon fiber wraps may improve their resistance. Encasement of
steel sections will inhibit local flange and web plate deformations that could
precipitate a section failure. Exterior column splices should be located as high
above grade level as practical and match the capacity of the column section.
Load-bearing walls do not benefit from clear-time effects as columns do and
therefore collect the full intensity of the reflected blast pressure pulse.
Nevertheless, reinforced concrete load-bearing walls are particularly effective
if adequately reinforced. Fully grouted masonry walls, on the other hand, are
more brittle and seismic levels of reinforcement greatly increase the ductility
and performance of masonry walls. Continuous reinforced bond beams, with a
minimum of one #4 bar or equivalent, are required in the wall at the top and
bottom of each floor and roof level. Interior horizontal ties are required in
the floors perpendicular to the wall. The ties are equivalent to a #4 bent bar
at a maximum spacing of 16 inches that extends into the slab and the wall the
greater of the development length of the bar or 30 inches. Vertical ties are
required from floor to floor at columns, piers, and walls. The ties should be
equivalent to a #4 bar at a maximum spacing of 16 inches coinciding with the
horizontal ties. The ties should be continuous through the floor and extend into
the wall above and below the floor the greater of the development length of the
bar or 30 inches. Partition walls surrounding critical systems or isolating
areas of internal threat, such as lobbies, loading docks, and mailrooms, require
fully grouted reinforced masonry construction. It is particularly difficult to
extend the reinforcement to the full height of the partition wall and develop
the reaction forces. Reinforced bond beams are required as for load-bearing
walls.
Flat roof systems are exposed to the incident blast pressures that diffuse over
the top of the building, causing complex patterns of shadowing and focusing on
the surface. Subsequent negative phase effects may subject the pre-weakened roof
systems to low intensity, but long duration suction pressures; therefore,
light­weight roof systems may be susceptible to uplift effects. Two-way beam
slab systems are preferred for reinforced concrete construc­tion and metal deck
with reinforced concrete fill is preferred for steel frame construction. Both of
these roof systems provide the required mass, strength, and continuity to resist
all phases of blast loading. The performance of conventional precast concrete
plank systems depends to a great extent on the connection details, and these
connections need to be detailed to provide continuity. Flat slab and flat plate
construction requires continuous bottom rein­forcement in both directions to
improve the integrity and special details at the columns to prevent a punching
shear failure. Post-tensioned slab systems are particularly problematic because
the cable profile is typically designed to resist the predominant patterns of
gravity load and the system is inherently weak in response to load reversals.
2.4.2 Façade and internal partitions
The building’s façade is its first real defense against the effects of a bomb
and is typically the weakest component that will be subjected to blast
pressures. Debris mitigating façade systems may be designed to provide a
reasonable level of protection to a low or moderate intensity threat; however,
façade materials may be locally overwhelmed in response to a low intensity short
stand-off detonation or globally overwhelmed in response to a large intensity
long stand-off detonation. As a result, it is unreasonable to design a façade to
resist the actual pressures resulting from the design level threat everywhere
over the surface of the building. In fact, successful performance of the blast-
resistant façade may be defined as throwing debris with less than high hazard
velocities. This is particularly true for the glazed fenestration. The peak
pressures and impulses that are used to select the laminated glazing makeup are
typically established such that no more than 10 percent of the glazed
fenestration will produce debris that is propelled with high hazard velocities
into the occupied space in response to any single detonation of the design level
threat. The definitions of high hazard velocities were adapted from the United
Kingdom hazard guides and correspond to debris that is propelled 10 feet from
the plane of the glazing and strikes a witness panel higher than 2 feet above
the floor. Similarly, a medium level of hazard corresponds to debris that
strikes the witness panel no higher than 2 feet above the floor. A low level of
hazard corresponds to debris that strikes the floor no farther than 10 feet from
the plane of the glazing and a very low level of hazard corresponds to debris
that strikes the floor no farther than 3.3 feet from the plane of the glazing.
Glass hazard response software was developed for the U.S. Army Corps of
Engineers, the General Services Administration, and the Department of State to
determine the performance of a wide variety of glazing systems in response to
blast loading. These simplified single-degree-of-freedom dynamic analyses
account for the strength of the glass prior to cracking and the post-damage
capacity of the laminated interlayers. While many of these glass hazard response
software remain restricted, the American Society for Testing and Materials
(ASTM) 2248 relates the design of glass to resist blast loading to an equivalent
3-second equivalent wind load.
In order for the glazing to realize its theoretical capacity, it must be
retained by the mullions with an adequately sized bite, by means of a structural
silicone adhesive, or a combination of the two. Furthermore, in order for the
mechanical bite and silicone adhesive to be effective, the mullion deformations
over the length of the lite must be limited (see Figure 2-13). Unfortunately,
the maximum extent of deformation that the mullion may sustain prior to
dislodging the glass is poorly defined. A conservative limit of 2 degrees is
often assumed for typical protective glazing systems; however, advanced
analytics may justify a significantly greater mullion deformation limit.
Mullions must therefore be able to accept the reaction forces from the edges of
the glazed elements and remain intact and attached to the building. Analyses of
mullion deformations and anchorage details are required to demonstrate the safe
performance of the glazed fenestration.
[Begin figure]
Figure 2-13: Protective façade design. An illustration showing a façade design.
- Window of protected spaces may be bullet-resistant
- Strong attachment to secure mullion
- Insulated glazing with laminated inner lite
- Window adhered to the frame with structural silicone sealant to keep fractured
window in frame
- Strengthened mullion system must be stronger than glass to hold fractured
window in place
[End figure]
Curtainwall systems are inherently lightweight and flexible façade systems;
however, well designed curtainwall systems demonstrated, through explosive
testing, considerable resilience in response to blast loading. Furthermore, the
glazed components are subjected to less intense loads as their flexible supports
deform in response to the blast pressures. A multi-degree-of-freedom model of
the façade will determine the accurate interaction of the individual mullions
and the phasing of the interconnecting forces. Because all response calculations
must be dynamic and inelastic, the accurate representation of the phasing of
these forces may significantly affect the performance. Curtainwall anchors are
attached directly to the floor slabs where the large lateral loads may be
transferred directly through the diaphragms into the lateral load-resisting
systems.
Façade systems may contain combinations of glazing, metal panels, precast
concrete, or stone panels. Metal panels provide little inertial resistance, but
are capable of developing large inelastic deformations. The fasteners that
attach these panels to the mullions or metal studs must be designed to transfer
the large membrane forces. Stone panels provide significant inertial resistance,
but are relatively brittle and have little strength beyond their modulus of
rupture. Stud wall systems that restrain these façade panels may deform within
acceptable levels and develop a membrane stiffening capacity, and strain energy
methods may be used to calculate their response. However, the anchorage of the
studs to the floor and ceiling slabs are likely to limit the forces they can
develop.
Precast panels may easily be designed to provide inelastic deformation in
response to the design level threats. However, the design of their anchorage to
hold them to the building during both the direct loading and subsequent rebound
phase require more robust details. Because the primary load carrying elements
may buckle in response to the large collected forces, precast panels are
attached directly to the floor slabs where the forces may be transferred through
the diaphragms to the lateral load-resisting elements. Where mullions are
attached within punched out openings in precast panels, the spacing of the
anchorages will determine the span of the mullions and the force each anchorage
is required to resist. Embedded anchors within the precast panels will be
re­quired to accept these anchorage forces.
Fully grouted and reinforced CMU façades may be designed to accept the large
lateral loads produced by blast events; however, it is often difficult to detail
them to transfer the reaction forces to the floor slabs. A continuous exterior
CMU wall that bears against the floor slabs may avoid many of the construction
and connection difficulties, but this is not typical construction practice.
Brick or stone veneer does not appreciably increase the strength of the CMU
wall, but the added mass increases its inertial resistance.
2.5 Existing Construction: Retrofitting Considerations
Although retrofitting existing buildings to include a shelter can be expensive
and disruptive to users, it may be the only available option. When retrofitting
existing space within a building is considered, data centers, interior
conference rooms, stairwells, and other areas that can be structurally and
mechanically isolated provide the best options. Designers should be aware that
an area of a building currently used for refuge may not necessarily be the best
candidate for retrofitting when the goal is to provide comprehensive protection.
An existing area that has been retrofitted to serve as a shelter is unlikely to
provide the same degree of protection as a shelter designed as new construction.
When existing space is retrofitted for shelter use, issues have arisen that have
challenged both designers and shelter operators. For example, glass and
unreinforced masonry façades are particularly vulnerable to blast loading.
Substantial stand-off distances are required for the unprotected structure and
these distances may be significantly reduced through the use of debris
mitigating retrofit systems. Furthermore, because blast loads diminish with
distance and incidence of blast wave to the loaded surface, the larger threats
at larger stand-off distances are likely to damage a larger percentage of façade
elements than the more localized effects of smaller threats at shorter stand-off
distances. Safe rooms that may be located within a building should therefore be
located in windowless spaces or spaces in which the window glazing was upgraded
with a fragment retention film (FRF).
2.5.1 Structure
The building’s lateral load-resisting system, the structural frame or shear
walls that resist wind and seismic loads, will be required to receive the blast
loads that are applied to the exterior façade and transfer them to the
building’s foundation. This load path is typically through the floor slabs that
act as diaphragms and interconnect the different lateral load-resisting
elements. The lateral load-resisting system for a building depends to a great
extent on the type of construction and region. In many cases, low-rise buildings
do not receive substantial wind and seismic forces and, therefore, do not
require substantial lateral load-resisting systems. Because blast loads diminish
with distance, a package sized explosive threat is likely to locally overwhelm
the façade, thereby limiting the force that may be transferred to the lateral
load-resisting system. However, the intensity of the blast loads that may be
applied to the building could exceed the design limits for most conventional
construction. As a result, the building is likely to be subjected to large
inelastic deformations that may produce severe cracks to the structural and
nonstructural partitions. There is little that can be done to upgrade the
existing structure to make it more ductile in response to a blast loading that
doesn’t require extensive renovation of the building; therefore, safe rooms
should be located close to the interior shear walls or reinforced masonry walls
in order to provide maximum structural support in response to these
uncharacteristically large lateral loads.
Unless the structure is designed to resist an extreme loading, such as a
hurricane or an earthquake, it is not likely to sustain extensive structural
damage without precipitating a progressive collapse. The effects of a satchel-
sized explosive in close contact to a column or a vehicle-borne explosive device
at a sidewalk’s distance from the façade may initiate a failure of a primary
structure that may propagate as the supported loads attempt to redistribute to
an adjoining structure. Transfer girders that create long span structures and
support large tributary areas are particularly susceptible to localized damage
conditions. As a result, safe rooms should not be located on a structure that is
either supported by or underneath a structure that is supported by transfer
girders unless the building is evaluated by a licensed professional engineer.
The connection details for multi-story precast structures should also be
evaluated before the building is used to house a safe room.
Nonstructural building components, such as piping, ducts, lighting units, and
conduits that are located within safe rooms must be sufficiently tied back to a
solid structure to prevent failure of the services and the hazard of falling
debris. To mitigate the effects of in-structure shock that may result from the
infilling of blast pressures through damaged windows, the nonstructural systems
should be located below the raised floors or tied to the ceiling slabs with
seismic restraints.
2.5.2 Façade and internal partitions
Safe rooms in existing buildings should be selected to provide the space
required to accommodate the building population and should be centrally located
to allow quick access from any location within the building, enclosed with
fragment mitigating partitions or façade, and within robust structural systems
that will resist collapse. These large spaces are best located at the lower
floors, away from a lightweight roof and exterior glazing elements. If such a
space does not exist within the existing building, the available spaces may be
upgraded to achieve as many of these attributes as possible. This will involve
the treatment of the exterior façade with fragment mitigating films, blast
curtains, debris catch systems, spray-on applications of elastopolymers to
unreinforced masonry walls, and hardening of select columns and slabs with
composite fiber wraps, steel jackets, or concrete encasements.
2.5.2.1 Anti-shatter Façade. The conversion of existing construc­tion to
provide blast-resistant protection requires upgrades to the most fragile or
brittle elements enclosing the safe room. Failure of the glazed portion of the
façade represents the greatest hazard to the occupants. Therefore, the exterior
glazed elements of the façade and, in particular, the glazed elements of the
designated safe rooms, should be protected with an FRF, also commonly known as
anti-shatter film (ASF), “shatter-resistant window film” (SRWF), or “security
film.” These materials consist of a laminate that will improve post-damage
performance of existing windows. Applied to the interior face of glass, ASF
holds the fragments of broken glass together in one sheet, thus reducing the
projectile hazard of flying glass fragments.
Most ASFs are made from polyester-based materials and coated with adhesives.
ASFs are available as clear, with minimal effects to the optical characteristics
of the glass, and tinted, which provides a variety of aesthetic and optical
enhancements and can increase the effectiveness of existing heating/cooling
systems. Most films are designed with solar inhibitors to screen out ultraviolet
(UV) rays and are available treated with an abrasion-resistant coating that can
prolong the life of tempered glass.1 (Footnote 1: Abrasions on the faces of
tempered glass reduce the glass strength.) However, over time, the UV absorption
damages the film and degrades its effectiveness.
According to published reports, testing has shown that a 7-mil thick film, or
specially manufactured 4-mil thick film, is the minimum thickness that is
required to provide hazard mitigation from blast. Therefore, a 4-mil thick ASF
should be utilized only if it has demonstrated, through explosive testing, that
it is capable of providing the desired hazard level response.
The application of security film must, at a minimum, cover the clear area of the
window. The clear area is defined as the portion of the glass unobstructed by
the frame. This minimum application, termed daylight installation, is commonly
used for retrofitting windows. By this method, the film is applied to the
exposed glass without any means of attachment or capture within the frame.
Application of the film to the edge of the glass panel, thereby extending the
film to cover the glass within the bite, is called an edge to edge installation
and is often used in dry glazing installations. Other methods of retrofit
application may improve the film perfor­mance, thereby reducing the hazards;
however, these are typically more expensive to install, especially in a retrofit
situation.
Although a film may be effective in keeping glass fragments together, it may not
be particularly effective in retaining the glass in the frame. ASF is most
effective when it is used with a blast tested anchorage system. Such a system
prevents the failed glass from exiting the frame (see Figure 2-14).
Figure 2-14: Mechanically attached anti-shatter film. An illustration showing a
section view of a window with glass on the outside and anti-shatter film on the
inside. A film anchoring device is attached to the frame, and film is extended
onto the frame surface.
The wet glazed installation, a system where the film is positively attached to
the frame, offers more protection than the daylight installation. This system of
attaching the film to the frame reduces glass fragmentation entering the
building. The wet glazing system utilizes a high strength liquid sealant, such
as silicone, to attach the glazing system to the frame. This method is more
costly than the daylight installation.
Securing the film to the frame with a mechanically connected anchorage system
further reduces the likelihood of the glazing system exiting the frame.
Mechanical attachment includes anchoring methods that employ screws and/or
batten strips that anchor the film to the frame along two or four sides. The
mechanical attachment method can be less aesthetically pleasing when compared to
wet glazing because additional framework is neces­sary and is more expensive
than the wet glazed installation.
Window framing systems and their anchorage must be capable of transferring the
blast loads to the surrounding walls. Unless the frames and anchorages are
competent, the effectiveness of the attached films will be limited. Similarly,
the walls must be able to withstand the blast loads that are directly applied to
them and accept the blast loads that are transferred by the windows. The
strength of these walls may limit the effectiveness of the glazing upgrades.
If a major rehabilitation of the façade is required to improve the mechanical
characteristics of the building envelope, a laminated glazing replacement is
recommended. Laminated glass consists of two or more pieces of glass permanently
bonded together by a tough plastic interlayer made of polyvinyl butyral (PVB)
resin. Once sealed together, the glass “sandwich” behaves as a single unit.
Annealed, heat strengthened, tempered glass, or polycarbonate glazing can be
mixed and matched between layers of laminated glass in order to design the most
effective lite for a given application. When fractured, fragments of laminated
glass tend to adhere to the PVB interlayer rather than falling free and
potentially causing injury.
Laminated glass can be expected to last as long as ordinary glass, provided it
is not broken or damaged in any way. It is very important that laminated glass
is correctly installed to ensure long life. Regardless of the degree of
protection required from the window, laminated glass needs to be installed with
adequate sealant to prevent water from coming in contact with the edges of the
glass. A structural sealant will adhere the glazing to the frame and allow the
PVB interlayer to develop its full membrane capacity. Similar to attached film
upgrades, the window frames and anchorages must be capable of transferring the
blast loads to the surrounding walls.
2.5.2.2 Façade Debris Catch Systems. Blast curtains are made from a variety of
materials, including a warp knit fabric or a polyethylene fiber. The fiber can
be woven into a panel as thin as 0.029 inch that weighs less than 1.5 ounces per
square foot. This fact dispels the myth that blast curtains are heavy sheets of
lead that completely obstruct a window opening and eliminate all natural light
from the interior of a protected building. The blast curtains are affixed to the
interior frame of a window opening and essentially catch the glass fragments
produced by a blast wave. The debris is then deposited on the floor at the base
of the window. Therefore, the use of these curtains does not eliminate the
possibility of glass fragments penetrating the interior of the occupied space,
but instead limits the travel distance of the airborne debris. Overall, the
hazard level to occupants is significantly reduced by the implementation of the
blast curtains. However, a person sitting directly adjacent to a window
outfitted with a blast curtain may still be injured by shards of glass in the
event of an explosion.
The main components of any blast curtain system are the curtain itself, the
attachment mechanism by which the curtain is affixed to the window frame, and
either a trough or other retaining mechanism at the base of the window to hold
the excess curtain material. The blast curtain with curtain rod attachment and
sill trough differ largely from one manufacturer to the next. The curtain
fabric, material properties, method of attachment, and manner in which they
operate all vary, thereby providing many options within the overall
classification of blast curtains. This fact makes blast curtains applicable in
many situations.
Blast curtains differ from standard curtains in that they do not open and close
in the typical manner. Although blast curtains are intended to remain in a
closed position at all times, they may be pulled away from the window to allow
for cleaning and blind or shade operation. However, the curtains can be rendered
ineffective if installed such that easy access would provide opportunity for
occupants to defeat their operation. The color and openness factor of the fabric
contributes to the amount of light that is transmitted through the curtains and
the see-through visibility of the curtains. Although the color and weave of
these curtains may be varied to suit the aesthetics of the interior décor, the
appearance of the windows is altered by the presence of the curtains.
The curtains may either be anchored at the top and bottom of the window frame or
anchored at the top only and outfitted with a weighted hem. The curtain needs to
be extra long, with the surplus either wound around a dynamic tension retainer
or stored in a reservoir housing. When an explosion occurs, the curtain feeds
out of the receptacle to absorb the force of the flying glass fragments. The
effectiveness of the blast curtains relies on their use and no protection is
provided when these curtains are pulled away from the glazing (see Figure 2-15).
Figure 2-15: Blast curtain system. An illustration showing the elevation view of
a blast curtain wall with the curtain, window frame, and glass with anti-shatter
film on the inside surface. There is a curtain rod attached to the wall above
the window and a curtain box attached to the wall below the window which holds
excess curtain and weighted curtain edge.
Rigid catch bar systems were designed and tested as a means of increasing the
effectiveness of filmed and laminated window upgrades. Anti-shatter film and
laminated glazing are designed to hold the glass shards together as the window
is damaged; however, unless the window frames and attachments are upgraded as
well to withstand the capacity of the glazing, this retrofit will not prevent
the entire sheet from flying free of the window frames. The rigid catch bars
intercept the filmed or laminated glass and disrupt their flight; however, they
are limited in their effectiveness, tending to break the dislodged façade
materials into smaller projectiles.
Rigid catch systems collect huge forces upon impact and require considerable
anchorage into a very substantial structure to prevent failure. If either the
attachments or the supporting structure are incapable of restraining the forces,
the catch system will be dislodged and become part of the debris. Alternatively,
the debris may be sliced by the rigid impact and the effectiveness of the catch
bar will be severely reduced. Finally, the effectiveness of debris catch systems
are limited where double pane, insulated glazing units (IGUs) are used. Since
anti-shatter film or laminated glass is typically applied to only the inner
surface of an IGU, debris from the damaged outer lite could be blown past the
catch bar into the protected space.
Flexible catch bars can be designed to absorb a significant amount of the energy
upon impact, thereby keeping the debris intact and impeding their flight. These
systems may be designed to effectively repel the debris and inhibit their flight
into the occupied spaces; they also may be designed to repel the debris from
the failed glazing as well as the walls in which the windows are mounted. The
design of the debris restraint system must be strong enough to withstand the
momentum transferred upon impact and the connections must be capable of
transferring the forces to the supporting slabs and spandrel beams. However,
under no circumstances can the design of the restraint system add significant
amounts of mass to the structure that may be dislodged and present an even
greater risk to the occupants of the building.
Cables are extensively used to absorb significant amounts of energy upon impact
and their flexibility makes them easily adaptable to many situations. The
diameter of the cable, the spacing of the strands, and the means of attachment
are all critical in designing an effective catch system. These catch cable
concepts have been used by protective design window manufacturers as restraints
for laminated lites. The use of cable systems has long been recognized as an
effective means of stopping massive objects moving at high velocity. An
analytical simulation or a physical test is required to confirm the adequacy of
the cable catch system to restrain the debris resulting from an explosive event.
High performance energy absorbing cable catcher systems retain glass and frame
fragments and limit the force transmitted to the supporting structure. These
commercially available retrofit products consist of a series of ¼-inch diameter
stainless steel cables connected with a shock-absorbing device to an aluminum
box section, which is attached to the jambs, the underside of the header, and
topside of the sill. The energy absorbing characteristics allow the catch
systems to be attached to relatively weakly constructed walls without the need
for additional costly structural reinforcement. To reduce the possibility of
slicing the laminated glass, the cable may either be sheathed in a tube or an
aluminum strip may be affixed to the glass directly behind the cable.
2.5.2.3 Internal Partitions. Unreinforced masonry walls provide limited
protection against airblast due to explosions. When subjected to overload from
air blast, brittle unreinforced CMU walls will fail and the debris will be
propelled into the interior of the structure, possibly causing severe injury or
death to the occupants. This wall type has been prohibited for new construction
where protection against explosive threats is required. Existing unreinforced
CMU walls may be retrofitted with a sprayed-on polymer coating to improve their
air blast resistance. This innovative retrofit technique takes advantage of the
toughness and resiliency of modern polymer materials to effectively deform and
dissipate the blast energy while containing the shattered wall fragments.
Although the sprayed walls may shatter in a blast event, the elastomer material
remains intact and contains the debris.
The blast mitigation retrofit for unreinforced CMU walls consists of an interior
and optional exterior layer of polyurea applied to exterior walls and ceilings
(see Figure 2-16). The polyurea provides a ductile and resilient membrane that
catches and retains secondary fragmentation from the existing concrete block as
it breaks apart in response to an air blast wave. These fragments, if allowed to
enter the occupied space, are capable of producing serious injury or death to
occupants of the structure.
Figure 2-16: Spray-on elastomer coating. An illustration showing a CMU wall with
a partition wall attached. Polymer coating is applied to the wall and shows a
minimum 15cm overlap onto the frame, and a minimum 15 cm overlap onto partition
wall, floor, and ceiling.
In lieu of the elastomer, an aramid (Geotextile) debris catching system may be
attached to the structure by means of plates bolted through the floor and
ceiling slabs (see Figure 2-17). Similar to the elastomer retrofit, the aramid
layer does not strengthen the wall; instead, it restrains the debris that would
otherwise be propelled into the occupied spaces.
Figure 2-17: Geotextile debris catch system. An illustration showing the debris
catching system.
Alternatively, an unreinforced masonry wall may be upgraded with an application
of shotcrete sprayed onto the wall with a welded wire fabric. This method
supplements the tensile capacity of the existing wall and limits the extent of
debris that might be propelled into the protected space. Steel sections may also
be in­stalled up against existing walls to reduce the span and provide an
alternate load transfer to the floor diaphragms. Load-bearing masonry walls
require additional redundancy to prevent the initiation of a catastrophic
progression of collapse. Therefore, the fragment protection that may be provided
by a spray-on elastopolymer, a fabric spall shield, or a metal panel must be
supplemented with structural supports that can sustain the gravity loads in the
event of excessive wall deformation. The design of stiffened steel-plate wall
systems to withstand the effects of explosive loading is one way of achieving
such redundancy and fragment protection. These load-bearing wall retrofits
require a more stringent design, capable of resisting lateral loads and the
transfer of axial forces. Stiffened wall panels, consisting of steel plates to
catch the debris and welded tube sections spaced some 3 feet on center to
supplement the gravity load carrying capacity of the bearing walls, must be
connected to the existing floor and ceiling slabs by means of base plates and
anchor bolt connectors (see Figure 2-18).
Figure 2-18: Stiffened wall panels. An illustration showing a hollow structural
section, and a detail of the connection to the concrete slab. This is done with
a base plate, shims as required and anchor bolts.
A steel stud wall construction technique may also be used for new buildings or
the retrofit of existing structures requiring blast resistance. Commercially
available 18-gauge steel studs may be attached web to web (back to back) and 16-
gauge sheet metal may be installed outboard of the steel studs behind the
cladding (see Figure 2-19). While the wall absorbs a considerable amount of the
blast energy through deformation, its connection to the surrounding structure
must develop the large tensile reaction forces. In order to prevent a premature
failure, these connections should be able to develop the ultimate capacity of
the stud in tension. Ballistic testing of various building cladding materials
requires a nominal 4-inch thickness of stone, brick, masonry, or concrete.
Forced entry protection requires a ¼-inch thick layer of A36 steel plate that is
behind the building’s veneer and welded or screwed to the steel stud framing in
lieu of the 16-gauge sheet metal.
Figure 2-19: Metal stud blast wall. A photo showing a steel stud wall
construction technique.
Internal installations require an interstitial sheathing of ½-inch A36 steel
plate. Regardless whether a ¼-inch steel plate or a 16-gauge sheet metal is
used, the interior face of the stud should be finished with a steel-backed
composite gypsum board product.
2.5.2.4 Structural Upgrades. Conventionally designed columns may be vulnerable
to the effects of explosives, particularly when placed in contact with their
surface. Stand-off elements, in the form of partitions and enclosures, may be
designed to guarantee a minimum stand-off distance; however, this alone may not
be sufficient. Additional resistance may be provided to reinforced concrete
structures by means of a steel jacket or a carbon fiber wrap that effectively
confines the concrete core, thereby increasing the confined strength and shear
capacity of the column, and holds the rubble together to permit it to continue
carrying the axial loads (see Figure 2-20). The capacity of steel flanged
columns may be increased with a reinforced concrete encasement that adds mass to
the steel section and protects the relatively thin flange sections. The details
for these retrofits must be designed to resist the specific weight of explosives
and stand-off distance.
Figure 2-20: Steel jacket retrofit detail. An illustration showing:
- Concrete core
- Sand blast concrete surfaces prior to jacketing
- Chip corners 1” and grind smooth
- 1” clear space around the columns filled with 5,000 psi non-shrink grout
- 1” radius bent plat
- 3/8” steel jacket
2.5.3 Checklist for Retrofitting Issues
A Building Vulnerability Assessment Checklist was developed for FEMA 426 and
FEMA 452 to help identify structural conditions that may suffer in response to
blast loading. Each building in consideration needs to be evaluated by a
professional engineer, experienced in the protective design of structures, to
determine the ability to withstand blast loading.
In addition, the following questions will help address key retrofitting issues.
Issues related to the retrofitting of existing refuge areas (e.g.,
hallways/corridors, bathrooms, workrooms, laboratory areas, kitchens, and
mechanical rooms) that should be considered include the following:
- The roof system. Is the roof system over the proposed refuge area structurally
independent of the remainder of the building? If not, is it capable of resisting
the expected blast, wind, and debris loads? Are there openings in the roof
system for mechanical equipment or lighting that cannot be protected during a
blast or high-wind event? It may not be reasonable to retrofit the rest of the
proposed shelter area if the roof system is part of a building that was not
designed for high-wind load requirements.
- The wall system. Can the wall systems be accessed so that they can be
retrofitted for resistance to blast and high-wind pressures and missile impact?
It may not be reasonable to retrofit a proposed shelter area to protect openings
if the wall systems (load-bearing or non-load-bearing) cannot withstand blast
and wind pressures or cannot be retrofitted in a reasonable manner to withstand
blast or wind pressures and missile impacts.
- Openings. Are the windows and doors vulnerable to blast and wind pressures and
debris impact? Are doors constructed of impact-resistant materials (e.g., steel
doors) and secured with six points of connection (typically three hinges and
three latching mechanisms)? Are door frames constructed of at least 16-gauge
metal and adequately secured to the walls to prevent the complete failure of the
door/frame assemblies? Does the building rely on shutter systems for resistance
to the effects of hurricanes? There is often only minimal warning time before a
CBRE or tornado event; therefore, a shelter design that relies on manually
installed shutters is impractical. Automated shutter systems may be considered,
but they would require a protected backup power system to ensure that the
shutters are closed before an event.
2.6 Shelters and Model Building Types
This section will provide basic FEMA model building types to describe protective
design and structural systems for shelters in the most effective manner. This
section is based on FEMA 310, Handbook for the Seismic Evaluation of Buildings,
which is dedicated to instructing the design professional on how to determine if
a building is adequately designed and constructed to resist particular types of
forces. Graphics included in this section were prepared for FEMA 454, Designing
for Earthquakes: A Manual for Architects.
2.6.1 W1, W1a, and W2 Wood Light Frames and Wood Commercial Buildings
Small wood light frame buildings (<3,000 square feet) are single or multiple
family dwellings of one or more stories in height (see Figure 2-21). Building
loads are light and the framing spans are short. Floor and roof framing consists
of closely spaced wood joists or rafters on wood studs. The first floor framing
is supported directly on the foundation, or is raised up on cripple studs and
post and beam supports. The foundation consists of spread footings constructed
of concrete, concrete masonry block, or brick masonry in older construction.
Chimneys, when present, consist of solid brick masonry, masonry veneer, or wood
frame with internal metal flues. Lateral forces are resisted by wood frame
diaphragms and shear walls. Floor and roof diaphragms consist of straight or
diagonal wood sheathing, tongue and groove planks, or plywood. Shearwalls
consist of straight or diagonal wood sheathing, plank siding, plywood, stucco,
gypsum board, particle board, or fiber­board. Interior partitions are sheathed
with plaster or gypsum board.
Figure 2-21: W1 wood light frame <3,000 square feet. An illustration showing a
single or multiple family dwelling of one or more stories in height.
Large wood light frame buildings (>3,000 square feet) are multi-story, multi-
unit residences similar in construction to W1 buildings, but with open front
garages at the first story (see Figure 2-22). The first story consists of wood
floor framing on wood stud walls and steel pipe columns, or a concrete slab on
concrete or concrete masonry block walls.
Figure 2-22: W1a wood light frame >3,000 square feet. An illustration showing a
large wood light frame building which is a multi-story, multi-unit residence
similar in construction to W1 buildings, but with open front garages at the
first story.
Wood commercial or industrial buildings with a floor area of 5,000 square feet
or more carry heavier loads than light frame construction (see Figure 2-23). In
these buildings, the framing spans are long and there are few, if any, interior
walls. The floor and roof framing consists of wood or steel trusses, glulam or
steel beams, and wood posts or steel columns. Lateral forces are resisted by
wood diaphragms and exterior stud walls sheathed with plywood, stucco, plaster,
straight or diagonal wood sheathing, or braced with rod bracing. Large openings
for storefronts and garages, when present, are framed by post-and-beam framing.
Lateral force resistance around openings is provided by steel rigid frames or
diagonal bracing.
Figure 2-23: W2 wood commercial buildings. An illustration showing a wood
commercial or industrial building with a floor area of 5,000 square feet
or more carry heavier loads than light frame construction.
Light wood frame structures do not possess significant resistance to blast loads
although larger wood commercial buildings will be better able to accept these
lateral loads than light frame wood construction. These buildings are likely to
suffer heavy damage in response to 50 pounds of TNT at a stand-off distance of
20 to 50 feet. A shelter would best be located in a basement where the
protection to blast loading would be provided by the surrounding soil. Large
explosive detonations in close proximity to the building will not only destroy
the superstructure, but the effects of ground shock are likely to fail the
foundation walls as well; therefore, protected spaces should be located interior
to the building. Locating the shelter on the ground floor, for slab on grade
structures, provides the maximum number of floors between occupants and possible
roof debris. Debris catch systems may be installed beneath roof rafters of
single-story buildings; however, the effectiveness of the debris catch system
will be limited if the zone of roof damage is extensive.
Metal stud blast walls built within the existing building may be used to
supplement the enclosure; however, in order for these walls to develop their
resistance to lateral loads, they must be anchored to an existing structure.
Windows enclosing the selected shelter must either be laminated or treated with
an anti-shatter film. Either the laminated glass or the anti-shatter film should
be anchored to the surrounding wall with a system that can develop but not
overwhelm the capacity of the wall. A conservative estimate of the ultimate
capacity of an existing wall may be determined, in the absence of actual design
information, by scaling the code specified wind pressures with the appropriate
factor of safety.
2.6.2 S1, S2, and S3 Steel Moment Frames, Steel Braced Frames, and Steel Light
Frames
Steel moment frame and braced frame buildings with cast-in-place concrete slabs
or metal deck with concrete fill supported on steel beams, open web joists, or
steel trusses are well suited for a hardened shelter construction. Lateral
forces in steel moment frame buildings are resisted by means of rigid or semi-
rigid beam-column connections (see Figure 2-24). When all connections are
moment-resisting connections, the entire frame participates in lateral force
resistance. When only selected connections are moment-resisting connections,
resistance is provided along discrete frame lines. Columns are oriented so that
each principal direction of the building has columns resisting forces in strong
axis bending. Diaphragms consist of concrete or metal deck with concrete fill
and are stiff relative to the frames. Walls may consist of metal panel
curtainwalls, glazing, brick masonry, or precast concrete panels. When the
interior of the structure is finished, frames are concealed by ceilings,
partition walls, and architectural column furring. Foundations consist of
concrete spread footings or deep pile foundations.
Figure 2-24: S1 steel moment frames. An illustration showing a steel moment
frame and braced frame building.
Lateral forces in steel braced frame buildings are resisted by tension and
compression forces in diagonal steel members (see Figure 2-25). When diagonal
brace connections are concentric to beam column joints, all member stresses are
primarily axial. When diagonal brace connections are eccentric to the joints,
members are subjected to bending and axial stresses. Diaphragms consist of
concrete or metal deck with concrete fill and are stiff relative to the frames.
Walls may consist of metal panel curtainwalls, glazing, brick masonry, or
precast concrete panels. When the interior of the structure is finished, frames
are concealed by ceilings, partition walls, and architectural furring.
Foundations consist of concrete spread footings or deep pile foundations.
Figure 2-25: S2 steel braced frames. An illustration showing lateral forces in
steel braced frame building.
Light frame steel structures are pre-engineered and prefabricated with
transverse rigid steel frames (see Figure 2-26). They are one-story in height
and the roof and walls consist of lightweight metal, fiberglass, or cementitious
panels. The frames are designed for maximum efficiency and the beams and columns
consist of tapered, built-up sections with thin plates. The frames are built in
segments and assembled in the field with bolted or welded joints. Lateral forces
in the transverse direction are resisted by the rigid frames. Lateral forces in
the longitudinal direction are resisted by wall panel shear elements or rod
bracing. Diaphragm forces are resisted by untopped metal deck, roof panel shear
elements, or a system of tension-only rod bracing.
Figure 2-26: S3 steel light frames. An illustration showing a light frame steel
structures which is pre-engineered and prefabricated with a transverse rigid
steel frame.
Steel moment frame structures provide excellent ductility and redundancy in
response to blast loading. Steel braced frames may similarly be designed to
resist high intensity blast loads; however, they are less effective in resisting
the progression of collapse following the loss of a primary load-bearing
element. As a result, first floor steel columns of existing buildings may be
concrete encased and first floor splices may be reinforced to increase their
resistance to local failure that could precipitate a progression of collapse.
The exterior façade represents the most fragile element and is likely to be
severely damaged in response to an exterior detonation. Debris may be minimized
by means of reinforced masonry, sufficiently detailed precast panels, or
laminated glass façade. Nevertheless, a shelter within steel frame buildings
would best be located within interior space or a building core. Hardened
interior partitions may easily be constructed and anchored to existing floor
slabs, and lightweight metal gauge walls may be used to retrofit existing
buildings. Metal deck roofs with rigid insulation supported by bar joist
structural elements possess minimal resistance to blast pressures. The
additional mass, stiffness, and strength of metal deck roofs with concrete fill
make them much better able to resist the effects of direct blast loading and the
subsequent rebound. Therefore, lightweight roofs of light frame steel structures
are likely to be severely damaged in response to any sizable blast loading and a
shelter should either be located in the basement or as interior to the building
(as far from the exterior façade) as possible.
2.6.3 S4 and S5 Steel Frames with Concrete Shearwalls and Infill Masonry Walls
Steel frame buildings with concrete or infill masonry shear walls with cast-in-
place concrete slabs or metal deck with concrete fill supported on steel beams,
open web joists, or steel trusses are well suited for a hardened shelter
construction. When lateral forces are resisted by cast-in-place concrete shear
walls, the walls carry their own weight. In older construction, the steel frame
is designed for vertical loads only. In modern dual systems, the steel moment
frames are designed to work together with the concrete shear walls in proportion
to their relative rigidity (see Figure 2-27). In the case of a dual system, the
walls should be evaluated under this building type and the frames should be
evaluated under S1 steel moment frames. Diaphragms consist of concrete or metal
deck with or without concrete fill. The steel frame may provide a secondary
lateral-force-resisting system, depending on the stiffness of the frame and the
moment capacity of the beam-column connections.
Figure 2-27: S4 steel frames with concrete shearwalls. An illustration showing a
modern dual systems.
Steel frames with infill masonry walls is an older type of building construction
(see Figure 2-28). The walls consist of infill panels constructed of solid clay
brick, concrete block, or hollow clay tile masonry. Infill walls may completely
encase the frame members, and present a smooth masonry exterior with no
indication of the frame. The lateral resistance of this type of construction
depends on the interaction between the frame and infill panels. The combined
behavior is more like a shear wall structure than a frame structure. Solidly
infilled masonry panels form diagonal compression struts between the
intersections of the frame members. If the walls are offset from the frame and
do not fully engage the frame members, the diagonal compression struts will not
develop. The strength of the infill panel is limited by the shear capacity of
the masonry bed joint or the compression capacity of the strut. The post-
cracking strength is determined by an analysis of a moment frame that is
partially restrained by the cracked infill. The diaphragms consist of concrete
floors and are stiff relative to the walls.
Figure 2-28: S5 steel frames with infill masonry walls. An illustration showing
a steel framed building with infill masonry walls.
Steel frame structures with either concrete shear walls or infill masonry walls
are not moment connected; therefore, the frame is more vulnerable to collapse
resulting from the loss of a column. As a point of reference, steel moment frame
buildings with lightly reinforced CMU infill walls are likely to suffer heavy
damage in response to 500 pounds of TNT at a stand-off distance of 50 feet or
less. The first floor steel columns of existing buildings may be concrete
encased and first floor splices may be reinforced to increase their resistance
to local failure that could precipitate a progression of collapse. The exterior
façade is likely to be damaged in response to an exterior detonation and debris
may be minimized by means of reinforced masonry, sufficiently detailed precast
panels, or laminated glass façade. Nevertheless, a shelter within these
buildings would best be located within interior space or a building core,
preferably enclosed on one or more sides by the shear walls. Existing masonry
infill walls may be retrofitted to supplement existing reinforcement by either
grouting cables within holes cored within the walls or with a spray-on
application of a shotcrete and welded wire fabric or a polyurea debris catch
membrane. Alternatively, hardened interior partitions may easily be constructed
and anchored to existing floor slabs, and light­weight metal stud walls may be
used to retrofit existing buildings.
2.6.4 C1, C2, and C3 Concrete Moment Frames, Concrete and Infill Masonry
Shearwalls – type 1 Bearing Walls and Type 2 Gravity Frames
These buildings consist of a frame assembly of cast-in-place concrete beams and
columns. Floor and roof framing consists of cast-in-place concrete slabs,
concrete beams, one-way joists, two-way waffle joists, or flat slabs. Lateral
forces are resisted by concrete moment frames that develop their stiffness
through monolithic beam-column connections (see Figure 2-29). In older
construction, or in regions of low seismicity, the moment frames may consist of
the column strips of two-way flat slab systems. Modern frames in regions of
high seismicity have joint reinforcing, closely spaced ties, and special
detailing to provide ductile performance. This detailing is not present in older
construction. Foundations consist of concrete spread footings or deep pile
foundations.
Figure 2-29: C1 concrete moment frames. An illustration showing a concrete
moment frame.
Concrete and infill masonry shearwall building systems have floor and roof
framing that consists of cast-in-place concrete slabs, concrete beams, one-way
joists, two-way waffle joists, or flat slabs. Floors are supported on concrete
columns or bearing walls. Lateral forces are resisted by cast-in-place concrete
shear walls or infill panels constructed of solid clay brick, concrete block, or
hollow clay tile masonry (see Figures 2-30, 2-31, and 2-32). In older
construction, cast-in-place shear walls are lightly reinforced, but often extend
throughout the building. In more recent construction, shear walls occur in
isolated locations and are more heavily reinforced with boundary elements and
closely spaced ties to provide ductile performance. The diaphragms consist of
concrete slabs and are stiff relative to the walls. Foundations consist of
concrete spread footings or deep pile foundations. The seismic performance of
infill panel construction depends on the interaction between the frame and
infill panels. The combined behavior is more like a shear wall structure than a
frame structure. If the infilled masonry panels are in line with the frame, they
form diagonal compression struts between the intersections of the frame members;
otherwise, the diagonal compression struts will not develop. The strength of the
infill panel is limited by the shear capacity of the masonry bed joint or the
compression capacity of the strut. The post-cracking strength is determined by
an analysis of a moment frame that is partially restrained by the cracked
infill. The shear strength of the concrete columns, after cracking of the
infill, may limit the semiductile behavior of the system.
Figure 2-30: C2 concrete shearwalls – type 1 bearing walls
Figure 2-31: C2 concrete shearwalls – type 2 gravity frames
Figure 2-32: C3 concrete frames with infill masonry shearwalls
Unless sited in a seismic zone, concrete frame structures are not typically
designed and detailed to develop large inelastic deformations and withstand
significant load reversals. As a point of reference, a building with 8-inch
thick reinforced concrete load-bearing exterior walls and interior columns is
likely to suffer heavy damage in response to 500 pounds of TNT at a distance of
70 feet or less. The exterior façade is likely to be damaged in response to an
exterior detonation and debris may be minimized by means of reinforced masonry,
sufficiently detailed precast panels, or laminated glass façade. Nevertheless,
a shelter within concrete frame and shearwall buildings would best be located
within interior space or a building core, preferably enclosed on one or more
sides by the shear walls. Existing masonry infill walls may be retrofitted to
supplement existing reinforcement by either grouting cables within holes cored
within the walls or with a spray-on application of a shotcrete and welded wire
fabric or a polyurea debris catch membrane. Alternatively, hardened interior
partitions may easily be constructed and anchored to existing floor slabs, and
lightweight metal stud walls may be used to retrofit existing buildings.
2.6.5 PC1 and PC2 Tilt-up Concrete Shearwalls and Precast Concrete Frames and
Shearwalls
Tilt-up concrete buildings are one or more stories in height and have precast
concrete perimeter wall panels that are cast on site and tilted into place (see
Figure 2-33). Floor and roof framing consists of wood joists, glulam beams,
steel beams, open web joists, or precast plank sections. Framing is supported on
interior steel or concrete columns and perimeter concrete bearing walls. The
floors consist of wood sheathing, concrete over form deck, or composite concrete
slabs. Roofs are typically untopped metal deck, but may contain lightweight
concrete fill. Lateral forces are resisted by the precast concrete perimeter
wall panels. Wall panels may be solid, or have large window and door openings
that cause the panels to behave more as frames than as shear walls. In older
construction, wood framing is attached to the walls with wood ledgers.
Foundations typically consist of concrete spread footings or deep pile
foundations.
Figure 2-33: PC1 tilt-up concrete shearwalls. An illustration showing a tilt-up
concrete buildings are one or more stories in height.
Precast concrete frames and shearwalls consist of precast concrete planks, tees,
or double-tees supported on precast concrete girders and precast columns (see
Figure 2-34). Lateral forces are resisted by precast or cast-in-place concrete
shear walls. Diaphragms consist of precast elements interconnected with welded
inserts, cast-in-place closure strips, or reinforced concrete topping slabs.
Figure 2-34: PC2 precast concrete frames and shearwalls. An illustration showing
precast concrete frames and shearwalls.
Precast construction benefits from higher quality wall and frame components than
cast-in-place structures; however, it lacks the continuity of construction
present in these systems. The resistance blast loading depends, to a great
extent, on the mechanical connections between the components. Designers must
consider the blast loading effects when designing and detailing these
connections. A shelter would best be located in a basement where the protection
to blast loading would be provided by the surrounding soil. Large explosive
detonations in close proximity to the building will not only destroy the
superstructure, but the effects of ground shock are likely to fail the
foundation walls as well; therefore, protected spaces should be located interior
to the building. Locating the shelter in the basement, for slab on grade
buildings, provides the maximum number of floors between occupants and possible
roof debris. Debris catch systems may be installed beneath roof rafters of
single-story buildings; however, the effectiveness of the debris catch system
will be limited if the zone of roof damage is extensive.
Metal stud blast walls built within the existing building may be used to
supplement the enclosure; however, in order for these walls to develop their
resistance to lateral loads, they must be anchored to an existing structure.
Windows enclosing the selected shelter must either be laminated or treated with
an anti-shatter film. Either the laminated glass or the anti-shatter film should
be anchored to the surrounding wall with a system that can develop, but not
overwhelm the capacity of the wall. A conservative estimate of the ultimate
capacity of an existing wall may be determined, in the absence of actual design
information, by scaling the code specified wind pressures with the appropriate
factor of safety.
2.6.6 RM1 and RM2 Reinforced Masonry Walls with Flexible Diaphragms or Stiff
Diaphragms and Unreinforced Masonry (URM) Load-bearing Walls
These buildings have bearing walls that consist of reinforced brick or concrete
block masonry. Wood floor and roof framing consists of wood joists, glulam
beams, and wood posts or small steel columns. Steel floor and roof framing
consists of steel beams or open web joists, steel girders, and steel columns.
Lateral forces are resisted by the reinforced brick or concrete block masonry
shear walls. Diaphragms consist of straight or diagonal wood sheathing, plywood,
or untopped metal deck, and are flexible relative to the walls (see Figure 2-
35). Foundations consist of brick or concrete spread footings.
Figure 2-35: RM1 reinforced masonry walls with flexible diaphragms
Buildings with reinforced masonry walls and stiff diaphragms are similar to RM1
buildings, except the diaphragms consist of metal deck with concrete fill,
precast concrete planks, tees, or double-tees, with or without a cast-in-place
concrete topping slab, and are stiff relative to the walls (see Figure 2-36).
The floor and roof framing is supported on interior steel or concrete frames or
inte­rior reinforced masonry walls.
Figure 2-36: RM2 reinforced masonry walls with stiff diaphragms
Unreinforced load-bearing masonry buildings often contain perimeter bearing
walls and interior bearing walls made of clay brick masonry (see Figure 2-37).
In older construction, floor and roof framing consists of straight or diagonal
lumber sheathing supported by wood joists, on posts and timbers. In more recent
construction, floors consist of structural panel or plywood sheathing rather
than lumber sheathing. The diaphragms are flexible relative to the walls. When
they exist, ties between the walls and diaphragms consist of bent steel plates
or government anchors embedded in the mortar joints and attached to framing.
Foundations consist of brick or concrete spread footings. As a variation, some
URM buildings have stiff diaphragms relative to the unreinforced masonry walls
and interior framing. In older construction or large, multi-story buildings,
diaphragms may consist of cast-in-place concrete. In regions of low seismicity,
more recent construction consists of metal deck and concrete fill supported on
steel framing.
Figure 2-37: URM load-bearing walls
Unless sited in a seismic zone, reinforced masonry structures are not typically
detailed to develop significant inelastic deformations and withstand significant
load reversals. Unreinforced masonry structures are extremely brittle. As a
point of reference, a reinforced masonry building with 8-inch thick reinforced
CMU exterior walls is likely to suffer heavy damage in response to 500 pounds of
TNT at a distance of 150 feet or less. An unreinforced masonry building with
reinforced CMU pilasters will suffer heavy damage in response to 500 pounds of
TNT at a distance of 250 feet or less. At these loads, the structure supported
by the load-bearing masonry wall is likely to suffer localized collapse. Grout
and additional reinforcement may be inserted within the cores of existing
masonry walls; however, a stiffened steel panel provides the most effective way
to restrain the debris and assume the gravity loads following the loss of load
carrying capacity within the wall. A shelter within these buildings would best
be located within inte­rior space or a building core, preferably enclosed on one
or more sides by the shear walls.
2.6.7 Conclusions
Despite the various types of construction, the following protective measures may
be used to establish a hardened space that will limit the extent of debris
resulting from an explosive event. A shelter is best located within interior
space or a building core at the lowest levels of a building or on the ground
floor for a slab on grade structure. A debris catch system should be installed
beneath the roof rafters of a single-story building. The exterior façade should
be either reinforced masonry or precast panels and windows should either be
laminated or treated with an anti-shatter film that is anchored to the
surrounding walls. First floor steel columns may be concrete encased and first
floor splices may be reinforced. Existing masonry infill walls may be
retrofitted by either grouting cables within holes cored within the walls or
with a spray-on application of a shotcrete and welded wire fabric or a polyurea
debris catch membrane. Hardened interior partitions, such as metal stud blast
walls, may be used to enclose the shelter and these walls should be anchored to
an existing structure. A stiffened steel panel may be constructed interior to
existing load-bearing masonry walls.
2.7 Case Study: Blast-Resistant Safe Room
Consider the example of a safe room established in the stairwell of a multi-
story office building: it may be assumed the original construction did not
provide for reinforced masonry or reinforced concrete enclosures. To achieve the
greatest stand-off distance and isolate the safe room from a vehicle-borne
explosive threat, the stairwell should be interior to the structure. This will
provide the maximum level of protection from an undefined explosive threat.
Although it is common to place emergency stairs within the building core, one
can only reasonably expect a reinforced concrete or reinforced masonry stair
enclosure for a shearwall lateral resisting structural system. Due to the large
difference in weight and constructability, a stud wall with gypsum board stair
enclosure will be routinely used in lieu of reinforced masonry or concrete for
framed construction. The stair enclosures may therefore be designed or upgraded
to include 16-gauge sheet metal supported by 18-gauge steel studs that are
attached web to web (back to back). These walls must be adequately anchored to
the existing floor slabs to develop the plastic capacity of the studs acting
both in flexure and in tension. Alternatively, fully grouted reinforced masonry
stairwell enclosures, #4 bars in each cell, may be specified. The masonry walls
must be adequately anchored to the existing floor slabs to develop the ultimate
lateral resistance of the wall in order to transfer the reaction loads into the
lateral resisting system of the building. Doors to the stairway enclosures are
to be hollow steel or steel clad, such as 14-gauge steel doors with 20-gauge
ribs, with pressed steel frames; double doors should utilize a center stile.
Doors should open away from the safe room and be securely anchored to the wall
construction, locally reinforced around the door.
Any windows within the stairwell enclosures are to contain laminated glass,
utilizing 0.060 PVB, that is adhered within the mullions with a ½-inch bead of
structural silicone. The mullions are to be anchored into the surrounding walls
to develop the full capacity of the glazing materials. Alternatively, a 7-mil
anti-shatter film may be applied to existing windows and mechanically attached
to the surrounding mullions to develop the full capacity of the film. A wet
glazed attachment of the film may alternatively be applied; however, this
provides a less reliable bond to the existing mullions.
Floor slabs within an interior stairwell will be isolated from the most direct
effects of an exterior explosive event and will not be subjected to significant
uplift pressures resulting from an exterior explosive event. Nevertheless, for
new construction, floor slabs should be designed to withstand a net upward load
of magnitude equal to the dead load plus half the live load for the floor
system.
For new construction, the structural frames are to be sufficiently tied as to
provide alternate load paths to surrounding columns or beams in the event of
localized damage. These tie forces should, at a minimum, conform to the DoD
Unified Facilities Criteria (UFC) 4-023-03, Design of Buildings to Prevent
Progressive Collapse. For reinforced concrete structures, seismic hooks and
seismic development lengths, as specified in Chapter 21 of the American Concrete
Institute (ACI) 318-02, should be used to anchor and develop steel
reinforcement. Internal tie reinforcement should be distributed in two
perpendicular directions and be continuous from one edge of the floor or roof to
the far edge of the floor or roof, using lap splices, welds, or mechanical
splices. In order to redistribute the forces that may develop, the internal ties
must be anchored to the peripheral ties at each end (see Figure 2-38). Steel
structures must be similarly tied, and each column must be effectively held in
position by means of horizontal ties in two orthogonal directions at each
principal floor level supported by that column.
Figure 2-38: Schematic of tie forces in a frame structure. An Illustration
showing the internal ties, the horizontal ties to columns, the peripheral ties,
and vertical ties.